Ceramic multilayer printed circuit boards have been used for many years for circuits for electrical apparatus, such as mainframe computers. Such printed circuit boards are made by casting glass and/or ceramic powders together with an organic binder into tapes, called green tapes. A metal circuit can be patterned onto the green tape by screen printing for example. Vias are formed in each green tape layer that are filled with a conductive material to connect the circuits of the various layers electrically. The green tape layers are then aligned and stacked, pressed together, and fired to burn off organic residues and sinter the glass, thereby forming a fired ceramic multilayer circuit board.
Originally ceramics such as alumina were used to form the green tape layers, but these ceramics require high firing temperatures, up to 1500.degree. C. This necessitated the use of refractory conductive metals, such as tungsten or molybdenum, to form the conductive circuit .patterns because such metals could withstand high firing temperatures without melting. More recently, lower temperature materials have been used, such as devitrifying glasses that can be fired at lower temperatures of 1000.degree. C. or less. Multilayer circuit boards made of these glass or glass-ceramic materials can be used with lower melting point and higher conductivity metals, such as silver, gold or copper. However, these printed circuit boards have the disadvantage that they are not as strong as alumina circuit boards.
Thus still more recently, low firing temperature glasses have been deposited on support substrates made of metal or ceramic to which the glasses will adhere. The support substrate can be of a thermally conductive material such as nickel, kovar, a ferrous/nickel/cobalt/maganese alloy, Invar.RTM., a ferronickel, low carbon steel, or Cu/kovar, a ferrous/nickel/cobalt/maganese alloy/Cu, Cu/Mo/Cu or Cu/Invar.RTM./Cu composites and the like, as well as thermally conductive ceramics such as aluminum nitride, silicon carbide, diamond and the like. These substrates impart added strength to the composite. A bonding glass, such as described in U.S. Pat. No. 5,277,724 to Prabhu, adheres the ceramic substrate formed from the green tape layers to the substrate. In addition, if chosen correctly, the bonding glass can reduce shrinkage of the green tape with respect to the metal substrate in at least the two lateral dimensions. Thus all of the shrinkage occurs in the thickness dimension only. This in turn reduces problems of alignment of the circuit patterns in the ceramic layers, and the via holes in the metal substrate after firing.
However, when it is desired to produce glass/ceramic multilayer ceramic circuit boards on both sides of the support substrate, the presence of the thermally and electrically conductive metal or ceramic core material between two circuit boards can cause short circuits. Thus the multilayer circuits on one side of the support substrate have been connected to the multilayer circuits on the other side of the support substrate by means of circuit traces or lines that extend around the periphery of the circuit board rather than through the support substrate. However, such peripheral traces are subject to damage or breakage during handling and assembly of the circuit boards into a module, for example, and in some cases the traces would have to be too long for an acceptable design. Such designs also increase wiring lengths and decrease interconnection density. Thus an improved method of permitting electrical connection between two ceramic multilayer circuit boards on both sides of a support substrate would be highly desirable.